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Added functions to perform 'octahedral' encoding and decoding of normals.

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See http://jcgt.org/published/0003/02/01/paper-lowres.pdf
This commit is contained in:
David Williams 2014-07-23 23:35:46 +02:00
parent dd5e34bc92
commit 4a42535f13
4 changed files with 135 additions and 52 deletions
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42
examples/Basic/main.cpp
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@ -50,14 +50,34 @@ void createSphereInVolume(SimpleVolume<uint8_t>& volData, float fRadius)
//And compute how far the current position is from the center of the volume
float fDistToCenter = (v3dCurrentPos - v3dVolCenter).length();
uint8_t uVoxelValue = 0;
// We actually want our volume to have high values in the center and low values as we move out, because our
// eath should be a solid sphere surrounded by empty space. If we invert the distance then this is a step in
// the right direction. We still have zero in the center, but lower (negative) values as we move out.
float density = -fDistToCenter;
//If the current voxel is less than 'radius' units from the center then we make it solid.
if(fDistToCenter <= fRadius)
{
//Our new voxel value
uVoxelValue = 255;
}
// By adding the 'planetRadius' we now have a function which starts at 'planetRadius' and still decreases as it
// moves out. The function passes through zero at a distance of 'planetRadius' and then continues do decrease
// as it gets even further out.
density += fRadius;
// Ideally we would like our final density value to be '255' for voxels inside the planet and '0' for voxels
// outside the planet. At the surface there should be a transition but this should occur not too quickly and
// not too slowly, as both of these will result in a jagged appearance to the mesh.
//
// We probably want the transition to occur over a few voxels, whereas it currently occurs over 255 voxels
// because it was derived from the distance. By scaling the density field we effectivly compress the rate
// at which it changes at the surface. We also make the center much too high and the outside very low, but
// we will clamp these to the corect range later.
//
// Note: You can try commenting out or changing the value on this line to see the effect it has.
density *= 50;
// Until now we've been defining our density field as if the threshold was at zero, with positive densities
// being solid and negative densities being empty. But actually Cubiquity operates on the range 0 to 255, and
// uses a threashold of 127 to decide where to place the generated surface. Therefore we shift and clamp our
// density value and store it in a byte.
density += 127;
uint8_t uVoxelValue = (uint8_t)(clamp(density, 0.0f, 255.0f));
//Wrte the voxel value into the volume
volData.setVoxelAt(x, y, z, uVoxelValue);
@ -74,12 +94,12 @@ int main(int argc, char *argv[])
openGLWidget.show();
//Create an empty volume and then place a sphere in it
SimpleVolume<uint8_t> volData(PolyVox::Region(Vector3DInt32(0,0,0), Vector3DInt32(63, 63, 63)));
createSphereInVolume(volData, 30);
SimpleVolume<uint8_t> volData(PolyVox::Region(Vector3DInt32(0,0,0), Vector3DInt32(31, 31, 31)));
createSphereInVolume(volData, 15);
// Extract the surface for the specified region of the volume. Uncomment the line for the kind of surface extraction you want to see.
auto mesh = extractCubicMesh(&volData, volData.getEnclosingRegion());
//auto mesh = extractMarchingCubesMesh(&volData, volData.getEnclosingRegion());
//auto mesh = extractCubicMesh(&volData, volData.getEnclosingRegion());
auto mesh = extractMarchingCubesMesh(&volData, volData.getEnclosingRegion());
// The surface extractor outputs the mesh in an efficient compressed format which is not directly suitable for rendering. The easiest approach is to
// decode this on the CPU as shown below, though more advanced applications can upload the compressed mesh to the GPU and decompress in shader code.

38
examples/common/example.frag
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@ -1,19 +1,19 @@
#version 130
// Passed in from the vertex shader
in vec4 worldPosition;
in vec4 worldNormal;
// the color that gets written to the display
out vec4 outputColor;
void main()
{
// Again, for the purposes of these examples we cannot be sure that per-vertex normals are provided. A sensible fallback
// is to use this little trick to compute per-fragment flat-shaded normals from the world positions using derivative operations.
vec3 normal = normalize(cross(dFdy(worldPosition.xyz), dFdx(worldPosition.xyz)));
// We are just using the normal as the output color, and making it lighter so it looks a bit nicer.
// Obviously a real shader would also do texuring, lighting, or whatever is required for the application.
outputColor = vec4(abs(normal) * 0.5 + vec3(0.5, 0.5, 0.5), 1.0);
}
#version 130
// Passed in from the vertex shader
in vec4 worldPosition;
in vec3 worldNormal;
// the color that gets written to the display
out vec4 outputColor;
void main()
{
// Again, for the purposes of these examples we cannot be sure that per-vertex normals are provided. A sensible fallback
// is to use this little trick to compute per-fragment flat-shaded normals from the world positions using derivative operations.
vec3 normal = normalize(cross(dFdy(worldPosition.xyz), dFdx(worldPosition.xyz)));
// We are just using the normal as the output color, and making it lighter so it looks a bit nicer.
// Obviously a real shader would also do texuring, lighting, or whatever is required for the application.
outputColor = vec4(abs(worldNormal.xyz), 1.0);
}

45
examples/common/example.vert
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@ -1,20 +1,25 @@
#version 140
in vec4 position; // This will be the position of the vertex in model-space
// The usual matrices are provided
uniform mat4 cameraToClipMatrix;
uniform mat4 worldToCameraMatrix;
uniform mat4 modelToWorldMatrix;
// This will be used by the fragment shader to calculate flat-shaded normals. This is an unconventional approach
// but we use it in this example framework because not all surface extractor generate surface normals.
out vec4 worldPosition;
void main()
{
// Standard sequence of OpenGL transformations.
worldPosition = modelToWorldMatrix * position;
vec4 cameraPosition = worldToCameraMatrix * worldPosition;
gl_Position = cameraToClipMatrix * cameraPosition;
}
#version 140
in vec4 position; // This will be the position of the vertex in model-space
in vec3 normal;
// The usual matrices are provided
uniform mat4 cameraToClipMatrix;
uniform mat4 worldToCameraMatrix;
uniform mat4 modelToWorldMatrix;
// This will be used by the fragment shader to calculate flat-shaded normals. This is an unconventional approach
// but we use it in this example framework because not all surface extractor generate surface normals.
out vec4 worldPosition;
out vec3 worldNormal;
void main()
{
// Standard sequence of OpenGL transformations.
worldPosition = modelToWorldMatrix * position;
vec4 cameraPosition = worldToCameraMatrix * worldPosition;
worldNormal = normal;
gl_Position = cameraToClipMatrix * cameraPosition;
}

62
library/PolyVoxCore/include/PolyVoxCore/MarchingCubesSurfaceExtractor.h
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@ -50,7 +50,7 @@ namespace PolyVox
// Each component of the normal is encoded using 5 bits of this variable.
// The 16 bits are -xxxxxyyyyyzzzzz (note the left-most bit is currently
// unused). Some extra shifting and scaling is required to make it signed.
uint16_t encodedNormal;
Vector2DFloat encodedNormal;
// User data
DataType data;
@ -64,7 +64,7 @@ namespace PolyVox
return result;
}
inline uint16_t encodeNormal(const Vector3DFloat& normal)
/*inline uint16_t encodeNormal(const Vector3DFloat& normal)
{
Vector3DFloat v3dNormal = normal;
v3dNormal += Vector3DFloat(1.0f, 1.0f, 1.0f);
@ -96,6 +96,64 @@ namespace PolyVox
result -= Vector3DFloat(1.0f, 1.0f, 1.0f);
return result;
}*/
// Returns ±1
float signNotZero(float v)
{
return v >= 0.0 ? +1.0 : -1.0;
}
Vector2DFloat signNotZero(Vector2DFloat v)
{
return Vector2DFloat((v.getX() >= 0.0) ? +1.0 : -1.0, (v.getY() >= 0.0) ? +1.0 : -1.0);
}
// Assume normalized input. Output is on [-1, 1] for each component.
Vector2DFloat float32x3_to_oct(Vector3DFloat v)
{
// Project the sphere onto the octahedron, and then onto the xy plane
Vector2DFloat p(v.getX(), v.getY());
p = p * (1.0f / (abs(v.getX()) + abs(v.getY()) + abs(v.getZ())));
float refX = ((1.0f - abs(p.getY())) * signNotZero(p.getX()));
float refY = ((1.0f - abs(p.getX())) * signNotZero(p.getY()));
Vector2DFloat ref(refX, refY);
// Reflect the folds of the lower hemisphere over the diagonals
return (v.getZ() <= 0.0) ? ref : p;
}
Vector3DFloat oct_to_float32x3(Vector2DFloat e)
{
Vector3DFloat v = Vector3DFloat(e.getX(), e.getY(), 1.0 - abs(e.getX()) - abs(e.getY()));
float refX = ((1.0f - abs(v.getY())) * signNotZero(v.getX()));
float refY = ((1.0f - abs(v.getX())) * signNotZero(v.getY()));
Vector2DFloat ref(refX, refY);
if (v.getZ() < 0.0f)
{
//v.xy = (1.0 - abs(v.yx)) * signNotZero(v.xy);
v.setX(refX);
v.setY(refY);
}
v.normalise();
return v;
}
inline Vector2DFloat encodeNormal(const Vector3DFloat& normal)
{
return float32x3_to_oct(normal);
}
inline Vector3DFloat decode(const Vector2DFloat& encodedNormal)
{
return oct_to_float32x3(encodedNormal);
}
/// Decodes a MarchingCubesVertex by converting it into a regular Vertex which can then be directly used for rendering.